Lead-free optical heavy flint glasses

ABSTRACT

The invention relates to lead-free optical glasses which have refractive indices n d  of between 1.65 and 1.80 and Abbe numbers ν d  of between 21 and 33 and possess the following composition (in % by weight, based on oxide): SiO 2  27-40; B 2 O 3  0-&lt;0.5; Al 2 O 3  0-6; Na 2 O 7-18; K 2 O 1-10; BaO 1-10; SrO 0-3; CaO 0.5-5; MgO 0-3; with BaO+SrO+CaO+MgO&lt;15; TiO 2  21-37; ZrO 2  0-7; Nb 2 O 5  5-17; WO 3  0.1-7.

The invention relates to lead-free optical glasses which have refractiveindices n_(d) of between 1.65 and 1.80 and Abbe numbers ν_(d) of between21 and 33. These glasses belong to the optical glass type consisting ofthe heavy flint glasses (HF).

Since, in recent years, the glass components PbO and As₂O₃ have beenconsidered to be environmentally polluting in public discussions, themanufacturers of optical apparatuses also require PbO-free andpreferably also As₂O₃-free glasses having the respective opticalproperties.

It is desirable to dispense with PbO also for the production of lightglass parts, i.e. of glasses having a low density.

It is as a rule not possible to reproduce the desired optical andtechnical glass properties influenced by PbO by simply replacing leadoxide by one or more components. Instead, new developments orwide-ranging changes in the glass composition are necessary.

The patent literature already includes some publications in whichlead-free glasses having optical values from said range are described.However, these glasses have a very wide range of disadvantages.

DE 32 164 51 A describes optical lightweight glasses having a refractiveindex n_(d) of >1.70 and an Abbe number ν_(d) of ≧22 and a density ρ of≦3.5 g/cm³. These glasses contain up to 3% by weight of B₂O₃, acomponent which is aggressive towards Pt. If such glasses are melted inPt crucibles or if they come into contact with other Pt components,which would improve the homogeneity and low bubble count of the glasses,they have a higher level of Pt impurities, with the result that theirtransmittance is adversely affected.

U.S. Pat. No. 3,589,918 describes an optical glass for lens systemscomprising the glass system SiO₂—K₂O—TiO₂—Sb₂O₃. The high Sb₂O₃ contentsof up to 45% by weight in this glass make the glass susceptible toseparation and heavy and adversely affect its transmittance so that itis not suitable for modern applications in optics.

JP 53-16718 A discloses glasses having high contents of divalent oxides(MO=MgO+CaO+SrO+BaO+ZnO 15-50% by weight) and relatively low contents ofTiO₂ (1-25% by weight). These glasses have Abbe numbers of between 30and 45. Owing to the high MO content, their stability to crystallizationis low.

JP 52-25812 A discloses TiO₂- and Nb₂O₅-glasses whose compositions varyover a wide range. According to the examples, the glasses have very high(25-45% by weight) or very low (5% by weight) of Nb₂O₅ contents. Thesame applies to the MO content (21 and 30% by weight and 0-5% by weight,respectively). These glasses having TiO₂ contents of up to 50% by weightare also not sufficiently stable to crystallization for economicalcontinuous production.

It is an object of the invention to provide lead-free optical glasseshaving a refractive index n_(d) of between 1.65 and 1.80 and an Abbenumber ν_(d) of between 21 and 33, which possess good melting andprocessing properties and have good chemical resistance, good stabilityto crystallization and a low density.

This object is achieved by glasses described in Patent claim 1.

The good fusibility meltability of the glasses is achieved by thebalanced proportions of fluxes (Na₂O, K₂O) to glass formers(SiO₂+optionally B₂O₃, Al₂O₃) in relation to the poorly melting highlyrefractive components (MO (BaO, CaO+optionally SrO, MgO), TiO₂, Nb₂O₅,WO₃+optionally ZrO₂)

The glasses contain 27 to 40% by weight of the main glass former SiO2.In the case of higher proportions, the desired high refractive indexwould not be reached and the fusibility would deteriorate; in the caseof lower proportions, the stability to crystallization and the chemicalresistance would be reduced. An SiO₂ content of at least 29% by weightis preferred, particularly preferably of at least 31% by weight of SiO₂.A content of not more than 36% by weight is particularly preferred.

For further stabilization, the glasses may contain up to 6% by weight ofAl₂O₃, preferably up to <3% by weight of Al₂O₃, and up to <0.5% byweight of B₂O₃. Higher proportions of glass formers would reduce thefusibility. Preferably, Al₂O₃ is dispensed with. It is a major advantagethat the B₂O₃ content can remain limited to said low proportions, sincethe aggressiveness of the glass melt is thus reduced, so that glassescontaining extremely small amounts of Pt impurities and hence havingvery high transmittances can be produced in Pt components.

In order to achieve the desired optical position of a heavy flint glass,relatively high proportions of highly refractive components arerequired. The proportion of the glass formers and of fluxes having a lowrefractive index (Na₂O, K₂O) is therefore limited. Preferably 69.5% byweight of SiO₂+Al₂O₃+B₂O₃+Na₂O +K₂O are not exceeded, and veryparticularly preferably the limit of this sum is max. 60.5% by weight.

In addition to the glass formers, the glasses contain a proportion offluxes which is sufficient for good fusibility. Thus, they contain atleast 8% by weight and not more than 28% by weight of Na₂O+K₂O, and inparticular 7-18% by weight of Na₂O and 1-10% by weight of K₂O. A fluxcontent of 12-26% by weight with 9-16% by weight of Na₂O and 3-10% byweight of K₂O is preferred, and at least 14% by weight of Na₂O+K₂O with10-15% by weight of Na₂O and 4-9% by weight of K₂O are particularlypreferred. A sum of Na₂O and K₂O of not more than 21% by weight is veryparticularly preferred.

The glasses contain the following highly refractive components:

They contain 1.5-<15% by weight of alkaline earth metal oxides,preferably 3.5-14, especially ≦11, particularly preferably ≦10, % byweight.

Specifically:

1-10% by weight of BaO, preferably 3-10, particularly preferably 3-8, %by weight

0.5-5% by weight of CaO, preferably 0.5-3% by weight

0-3% by weight of MgO, preferably 0-<2% by weight, preferably Mgo-free

0-3% by weight of Sro, preferably 0-<2% by weight, preferably SrO-free

The proportion of alkaline earth metal oxide is limited to said maximumcontent since a further increase would be possible only by reducing theglass former and flux content and would lead to crystallization effects,particularly since the further components which increase the refractiveindex are comparatively good nucleating agents. Said minimum contents ofthe alkaline earth metal oxides are necessary in order to establish thehigh refractive index and to stabilize the chemical resistance.

The glasses contain 21-37% by weight of TiO₂, preferably 23-35,particularly preferably 26-33, % by weight.

The glasses furthermore contain 5-17% by weight of Nb₂O₅, preferably >5,especially 7-15, particularly preferably ≦12, % by weight.

These two components form the basis of the high refractive index at thedesired Abbe number. An increase in the TiO₂ content would reduce theAbbe number excessively and also excessively increase the tendency tocrystallization. An increase in the Nb₂O₅ content would increase theAbbe number to a very excessive extent and slightly reduce therefractive index.

For stabilization to crystallization, the glasses may contain up to 7%by weight of ZrO_(2,) preferably <5% by weight. Preferably, the ZrO₂content replaces a corresponding part of the TiO₂ content, so thatpreferably the maximum sum of TiO₂+ZrO₂ is 37% by weight, in particular35% by weight.

A further increase of ZrO₂ would in turn lead to an increase in thetendency to crystallization; furthermore the optical position would beundesirably shifted.

By parallel use of different nucleating agents and crystal formers,namely TiO₂ in addition to Nb₂O₅ and optionally ZrO₂, the formation ofdefined crystals is impeded and it is possible to achieve the desiredexceptional refractive index by means of high proportions of thesecomponents without adding lead.

An important component is WO₃. It is present in an amount of 0.1 to 7%by weight in the glass and, in addition to the fine adjustment of theoptical position, serves for further reducing the tendency tocrystallization by its spatial coordination, which is unusual in thisglass system. This too impedes the formation of defined crystals. Inthis glass system, a higher WO₃ content would in turn undesirably shiftthe optical position. A WO₃ content of between 0.2 and 5% by weight ispreferred, particularly preferably between 0.2 and 4% by weight.

In order to improve the glass quality, one or more refining agents knownper se can be added in the customary amount to the mixture for refiningthe glass. Thus, the glass has a particularly good internal glassquality with respect to freedom from bubbles and freedom from stria.

If, instead of As₂O₃, for example, Sb₂O₃ is used as refining agent,preferably an amount of up to 1% by weight, which is possible withoutlosses in respect of the glass quality, the glasses which are lead-freeaccording to the invention are additionally arsenic-free.

The glasses may also contain, for example, up to 1% by weight of F⁻and/or up to 1% by weight of Cl⁻. F⁻ is added, for example, as KF orKHF₂. Cl⁻ is added, for example, as NaCl.

The glasses from said composition range have refractive indices n_(d) ofbetween 1.65 and 1.80 and Abbe numbers ν_(d) of between 21 and 33.Glasses from the composition ranges stated in each case as beingpreferred have refractive indices of n_(d) of between 1.68 and 1.79 andAbbe numbers ν_(d) of between 23 and 32. The refractive indices n_(d)and the Abbe numbers ν_(d) of glasses from the ranges stated as beingparticularly preferred are between 1.70 and 1.79 and between 24 and 28.

The entire disclosure of all applications, patents and publications,cited herein and of corresponding German application No. 10133763.9,filed Jul. 11, 2001 are incorporated by reference herein.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The following preferred specific embodiments are,therefore, to be construed as merely illustrative, and not limitative ofthe remainder of the disclosure in any way whatsoever.

In the foregoing and in the following examples, all temperatures are setforth uncorrected in degrees Celsius and, all parts and percentages areby weight, unless otherwise indicated.

EXAMPLES

Seven examples of glasses according to the invention were produced fromcustomary raw materials by melting.

Table 2 shows the respective composition (in % by weight, based onoxide), the refractive index n_(d), the Abbe number ν_(d), the partialdispersion in the blue range of the spectrum P_(g,F) and the anomalouspartial dispersion ΔP_(g,F), the density ρ[g/cm³], the coefficient ofthermal expansion α_(20/300) [10⁻⁶/K] and the glass transitiontemperature T_(g) [°C] of the glasses.

By way of explanation:

The partial dispersive power, the so-called relative partial dispersion,in the blue part of the spectrum is represented by the expression$P_{g,F} = {\frac{n_{g} - n_{F}}{n_{F} - n_{c}}.}$

By definition, the “normal line” obeys the equation

P _(g,F)=0.6438−0.001682·ν_(d)

in the blue range of the spectrum.

Glasses whose partial dispersion lies on this line are referred to as“normal glasses”.

In the case of glasses which have a dispersion behaviour deviating from“normal glasses”, the ordinal difference ΔP_(g,F) by which the relevantP_(g,F)−ν_(d) point is shifted relative to the “normal line”, is stated:

A coarse classification of these glasses having anomalous partialdispersion into two groups is usual, depending on whether P_(g,F) is“above” (positive partial dispersion: ΔP_(g,F)=pos.) or “below”(negative partial dispersion: ΔP_(g,F)=neg.) the “normal line”.

The glasses according to the invention were produced as follows: the rawmaterials for the oxides, preferably carbonates and nitrates, wereweighed out. The refining agent or agents was or were added, andthorough mixing was then carried out. The glass mixture was melted atabout 1350° C. in a continuous Pt melting unit, then refined at about1400° C. and thoroughly homogenized. At a pouring temperature of about1300° C., the glass was poured and was processed to give the desireddimensions.

Table 1 shows an example of melting.

TABLE 1 Example of melting for 100 kg of glass (calculated) Oxide % byweight Raw material Weight [kg] SiO₂ 35.0 SiO₂ 34.97 Na₂O 9.0 Na₂CO₃15.39 K₂O 5.0 K₂CO₃ 7.34 CaO 3.0 CaCO₃ 5.31 BaO 5.0 Ba(NO₃)₂ 0.34 BaCO₃6.18 TiO₂ 23.0 TiO₂ 23.11 Nb₂O₃ 15.0 Nb₂O₅ 15.00 WO₃ 5.0 WO₃ 5.00 Sb₂O₃0.1 Sb₂O₃ 0.10

The properties of the glasses obtained are shown in table 2, in example3.

TABLE 2 Glass compositions (in % by weight, based on oxide) andimportant properties: 1 2 3 4 5 6 7 SiO₂ 29.0 40.0 35.0 35.1 34.0 35.039.8 B₂O₃ 0.4 0.4 0.2 Al₂O₃ 6.0 2.5 6.0 Na₂O 16.0 9.0 9.0 12.0 11.1 13.012.3 K₂O 10.0 3.0 5.0 3.0 6.0 8.0 5.0 MgO 1.0 CaO 2.0 1.0 3.0 0.5 2.21.0 0.5 SrO 1.0 BaO 3.0 3.0 5.0 10.0 6.5 4.0 3.0 TiO₂ 23.0 34.0 23.028.0 29.5 29.0 23.0 ZrO₂ 3.0 Nb₂O₅ 7.0 7.0 15.0 10.0 9.5 7.0 7.0 WO₃ 2.03.0 5.0 1.0 0.7 0.5 0.2 Sb₂O₃ 0.1 0.1 0.1 0.1 0.1 0.1 0.1 n_(d) 1.68361.7816 1.7665 1.7700 1.7613 1.7251 1.6939 ν_(d) 31.26 23.82 26.49 26.1825.98 27.96 29.54 P_(g,F) 0.5964 0.6251 0.6128 0.6138 0.6126 0.60760.6056 ΔP_(g,F) (10⁻⁴) 52 214 135 140 131 109 115 ρ [g/cm³] 2.96 3.043.21 3.19 3.08 2.99 2.92 α_(20/300) [10⁻⁶/K] 13.5 8.4 9.5 10.0 10.5 11.39.8 Tg [° C.] 508 606 609 599 581 560 595

The glass composition range according to the invention offers a group oflead-free and, in a preferred embodiment, also As₂O₃-free opticalglasses of glass type HF having said optical data, which have otherproperties improved compared with the known glasses. The freedom of theglasses from lead not only is advantageous because of the environmentalprotection concept discussed but also has a positive effect on theirdensity and their glass transition temperature.

The glasses have the following advantages:

They have high chemical resistance; thus, they belong to the acidresistance class (ISO 8424) AR=2 or better and to the alkalineresistance class (ISO 10629) AR=2 or better, it being possible for therespective resistances to be 2.× or 1.×. The high chemical resistance ofthe glasses is important for their further processing, such as grindingand polishing.

The glasses have high stability to crystallization. It is thus possibleto produce the glasses in relatively large melting units, for example inan optical trough.

The glasses are easy to melt and can be readily processed.

With at least 500° C., they have relatively high glass transitiontemperatures Tg.

Their density ρ of not more than 3.4 g/cm³ is very low. This isparticularly remarkable since the low density is not realized throughhigh B₂O₃ contents.

The transmittance of the glasses in the visible range of the spectrum ishigh. Thus, the spectral pure transmittance τ_(i) is at the wavelengthλ=420 nm and a sample thickness of 25 mm (τ_(i420 nm;) 25 mm>75%).

With these properties, especially with their optical position, theirpartial dispersion P_(g,F) and their anomalous partial dispersionΔP_(g,F) and their transmittance, the glasses are outstandingly suitablefor use as optical elements (lenses, prisms) as well as optical fibresand image-transmitting fibres in the optical applications of imaging,projection, telecommunication and laser technology.

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

What is claimed is:
 1. A lead-free optical glass having a refractiveindex n_(d) of 1.65 to 1.80 and an Abbe number ν_(d) of 21 to 33comprising in % by weight, based on oxide SiO₂ 27-40 B₂O₃   0-<0.5 Al₂O₃0-6 Na₂O  7-18 K₂O  1-10 BaO  1-10 SrO 0-3 CaO 0.5-5   MgO 0-3 withBaO + SrO + CaO + MgO <15 TiO₂ 21-37 ZrO₂ 0-7 Nb₂O₅  5-17 WO₃ 0.1-7  

and optionally one or more refining agents.
 2. A lead-free optical glassaccording to claim 1, having a refractive index n_(d) of 1.68 to 1.79and an Abbe number ν_(d) of 23 to 32, comprising in % by weight, basedon oxide SiO₂ 29-40 B₂O₃   0-<0.5 Al₂O₃  0-<3 Na₂O  9-16 K₂O  3-10 BaO 3-10 SrO  0-<2 CaO 0.5-3   MgO  0-<2 with BaO + SrO + CaO + MgO ≦14TiO₂ 23-35 ZrO₂  0-<5 Nb₂O₅  7-15 WO₃ 0.2-5  

and optionally one or more refining agents.
 3. A lead-free optical glassaccording to claim 1, having a refractive index n_(d) of 1.70 to 1.79and an Abbe number ν_(d) of 24 to 28, comprising in % by weight, basedon oxide SiO₂ 31-36 B₂O₃   0-<0.5 Na₂O 10-15 K₂O 4-9 BaO 3-8 CaO 0.5-3  with BaO + CaO ≦10 TiO₂ 26-33 Nb₂O₅  7-12 WO₃ 0.2-4  

and optionally one or more refining agents.
 4. A lead-free optical glassaccording to claim 1, wherein the glass contains in % by weight Sb2O30-1 Cl— 0-1 F— 0-1


5. A lead-free optical glass according to claim 1, wherein the glass isfree of arsenic oxide, with the exception of unavoidable impurities. 6.A lead-free optical glass according to claim 1, having a density ν ofnot more than 3.4 g/cm³ and a glass transition temperature Tg of ≦500°C.
 7. A lead-free optical glass according to claim 1, wherein therefining agent is As₂O₃ or Sb₂O_(3.)
 8. A lead-free optical glassaccording to claim 7 containing up to 1% by weight of Sb₂O_(3.)
 9. Alens or prism comprising a lead-free optical glass according to claim 1.10. An optical fiber or an image-transmitting fiber comprising alead-free optical glass according to claim 1.